- UNDERSTANDING CAMERA LENS FLARE -

Lens flare is created when non-image forming light enters the lens and subsequently
hits the camera's film or digital sensor. This often appears as a characteristic
polygonal shape, with sides which depend on the shape of the lens diaphragm.
It can lower the overall contrast of a photograph significantly and is often
an undesired artifact, however some types of flare may actually enhance the
artistic meaning of a photo. Understanding lens flare can help you use
it--or avoid it--in a way which best suits how you wish to portray the final
image.

WHAT IT LOOKS LIKE

The above image exhibits tell-tale signs of flare in the upper right caused
by a bright sun just outside the image frame. These take the form of polygonal
bright regions (usually 5-8 sides), in addition to bright streaks and an overall
reduction in contrast (see below). The polygonal shapes vary in size and
can actually become so large that they occupy a significant fraction of the
image. Look for flare near very bright objects, although its effects can
also be seen far away from the actual source (or even throughout the image).

Flare can take many forms, and this may include just one or all of the polygonal
shapes, bright streaks, or overall washed out look (veiling flare) shown above.

BACKGROUND: HOW IT HAPPENS

All but the simplest cameras contain lenses which are actually comprised
of several "lens elements." Lens flare is caused by non-image light which
does not pass (refract) directly along its intended path, but instead reflects
internally on lens elements any number of times (back and forth) before finally
reaching the film or digital sensor.

Note: The aperture above is shown as being behind several lens
elements.

Lens elements often contain some type of anti-reflective coating which aims
to minimize flare, however no multi-element lens eliminates it entirely.
Light sources will still reflect a small fraction of their light, and this reflected
light becomes visible as flare in regions where it becomes comparable in intensity
to the refracted light (created by the actual image). Flare which appears
as polygonal shapes is caused by light which reflects off the inside edges of
the lens aperture (diaphragm), shown above.

Although flare is technically caused by internal reflections, this often
requires very intense light sources in order to become significant (relative
to refracted light). Flare-inducing light sources may include the sun,
artificial lighting and even a full moon. Even if the photo itself contains
no intense light sources, stray light may still enter the lens if it hits the
front element. Ordinarily light which is outside the angle of view does
not contribute to the final image, but if this light reflects it may travel
an unintended path and reach the film/sensor. In the visual example with
flowers, the sun was not actually in the frame itself, but yet it still caused
significant lens flare.

REDUCING FLARE WITH LENS HOODS

A good lens hood can nearly eliminate flare caused by stray light
from outside the angle of view. Ensure that this hood has a completely
non-reflective inner surface, such as felt, and that there are no regions which
have rubbed off. Although using a lens hood may appear to be a simple
solution, in reality most lens hoods do not extend far enough to block all stray
light. This is particularly problematic when using 35 mm lenses on a digital
SLR camera with a "crop factor," because these lens hoods were made for the
greater angle of view. In addition, hoods for zoom lenses can only be
designed to block all stray light at the widest focal length.

Petal lens hoods often protect better than non-petal (round) types.
This is because petal-style hoods take into account the aspect ratio of the
camera's film or digital sensor, and so the angle of view is greater in one
direction than the other.

If the lens hood is inadequate, there are some easy but less convenient workarounds.
Placing a hand or piece of paper exterior to the side of the lens which
is nearest the flare-inducing light source can mimic the effect of a proper
lens hood. On the other hand, it is sometimes hard to gauge when this
makeshift hood will accidentally become part of the picture. A more expensive
solution used by many pros is using adjustable bellows. This is
just a lens hood which adjusts to precisely match the field of view for a given
focal length.

Another solution to using 35 mm lenses and hoods on a digital SLR with
a crop factor is to purchase an alternative lens hood. Look for one
which was designed for a lens with a narrower angle of view (assuming this still
fits the hood mount on the lens). One common example is to use the EW-83DII
hood with Canon's 17-40 f/4L lens, instead of the one it comes with. The
EW-83DII hood works with both 1.6X and 1.3X (surprisingly) crop factors as it
was designed to cover the angle of view for a 24 mm lens on a full-frame 35
mm camera. Although this provides better protection, it is still only
adequate for the widest angle of view for a zoom lens.

Despite all of these measures, there
is no perfect solution. Real-world lens hoods cannot protect
against stray light completely since the "perfect" lens hood would
have to extend all the way out to the furthest object, closely following
the angle of view.

Unfortunately, the larger the lens hood the
better-- at least when only considering its light-blocking ability.
Care should still be taken that this hood does not block any of
the actual image light.

INFLUENCE OF LENS TYPE

In general, fixed focal length (or prime) lenses are less susceptible
to lens flare than zoom lenses. Other than having an inadequate
lens hood at all focal lengths, more complicated zoom lenses often have to contain
more lens elements. Zoom lenses therefore have more internal surfaces
from which light can reflect.

Wide angle
lenses are often designed to be more flare-resistant to bright light sources,
mainly because the manufacturer knows that these will likely have the sun within
or near the angle of view.

Modern high-end lenses typically contain better anti-reflective coatings.
Some older lenses made by Leica and Hasselblad do not contain any special coatings,
and can thus flare up quite significantly under even soft lighting.

MINIMIZING FLARE THROUGH COMPOSITION

Flare is thus ultimately under the control of the photographer, based on
where the lens is pointed and what is included within the frame.

Although photographers never like
to compromise their artistic flexibility for technical reasons,
certain compositions can be very effective at minimizing flare.
The best solutions are those where both artistic intent and technical
quality coexist.

One effective technique is to place objects within
your image such that they partially or completely obstruct any flare-inducing
light sources. The image on the left shows a cropped region
within a photo where a tree trunk partially obstructed a street
light during a long exposure. Even if the problematic light
source is not located within the image, photographing from a position
where that source is obstructed can also reduce flare.

The best approach is to of course shoot with the problematic
light source to your back, although this is usually either too limiting
to the composition or not possible. Even changing the angle
of the lens slightly can still at least change the appearance and
position of the flare.

VISUALIZING FLARE WITH THE DEPTH OF FIELD PREVIEW

The appearance and position of lens flare changes depending on the aperture
setting of the photo. The viewfinder image in a SLR camera represents
how the scene appears only when the aperture is wide open (to create the brightest
image), and so this may not be representative of how the flare will appear after
the exposure. The
depth of field preview button can be used to simulate what the flare will
look like for other apertures, but beware that this will also darken the viewfinder
image significantly.

The depth of field preview button is usually found at the base of the lens
mount, and can be pressed to simulate the streaks and polygonal flare shapes.
This button is still inadequate for simulating how "washed out" the final image
will appear, as this flare artifact also depends on the length of the exposure
(more on this later).

OTHER NOTES

Lens filters, as with lens elements, need to have a good anti-reflective
coating in order to reduce flare. Inexpensive UV, polarizing, and neutral
density filters can all increase flare by introducing additional surfaces which
light can reflect from.